This invention relates to the detection of defects in regular periodic patterns and particularly to an inspection system for detecting defects in such patterns wherein the elements of the patterns have variable periodicity. Although the present invention may be utilized to detect defects in many different types of regular periodic patterns, it hereinafter will be described with respect to the detection of defects in photographic master plates used in forming shadow masks of color television picture tubes.
Shadow masks used in color picture tubes are manufactured by a photolithography method in which a glass working plate with a suitable pattern on it is pressed against a photoresist coated steel sheet while the photoresist material is exposed by a suitable light source. During the many repetitions of this procedure, the pattern on the glass plate can become damaged. Either small portions of the black dots or bars which form the pattern can become torn off, or dirt particles can become pressed into the pattern. Masks made from defective plates will be faulty and must be discarded. Therefore, there is a need to periodically inspect the glass working plates.
Small defects of the dark dots or bars superimposed on a light background are very difficult to see when the pattern is viewed without magnification. This is in marked contrast to the negative pattern, such as is formed by the finished shadow mask, where enlarged or extra holes are quite visible, particularly to a trained observer. One way of enhancing visibility of some defects in a working plate is to superimpose a closely matched negative plate. Missing parts of the pattern will then show up as bright transmission spots. If the negative is shifted, extra dark spots can be given an enhanced visibility. However, this technique has a limited usefulness. For a satisfactory complete inspection, up to now it has been necessary to visually scan the plate under magnification, a process taking about 2 hours per plate.
A problem involved in automating the inspection of regular periodic patterns occurs when the pattern periodicity that is the element-to-element spacing of the pattern, varies over the pattern. The present invention solves this problem while providing an essentially automatic inspection system.